1. Field of the Invention

The present invention relates to a dielectric filter having a plurality of resonators formed in a dielectric block. More specifically, the invention relates to a dielectric filter whose spurious characteristics can easily be improved by modifying the pattern of the input/output electrode on its open end face of the dielectric block.
2. Description of the Related Art
Mobile communication devices such as cellular phones use various dielectric filters including duplexer which comprises a transmission filter and a reception filter. A conventional dielectric filter in use has a plurality of coaxial dielectric resonators having resonation holes inside a dielectric block as shown in FIG. 7 (e.g., JP(B)-H3-40962, page 8 and FIG. 2).
Referring to FIG. 7, the dielectric filter has a dielectric block 1 with an approximately hexahedral shape which has a first end face 1a and a second end face 1b disposed in opposite side to the first end face, a plurality of through holes 6 formed in the dielectric block 1 which extend from the first end face 1a to the second end face 1b, an inner conductor 3 formed on the inner wall of each through hole 6, a resonator coupling electrode 13 extending on the first end face (open end face) 1a from the inner conductor 3, an outer conductor 5 formed on the outer surface of the dielectric block 1, and an input/output electrode 4. The resonator coupling electrodes 13 couple the individual resonators to one another. Particularly, the resonator coupling electrodes 13 formed at the resonator at the input or output stage is used for external coupling with the input/output electrode 4. The input/output electrode 4 is connected to an external circuit on a mount surface A of the dielectric block 1, which is one of lateral faces each being disposed between an edge of the first end face 1a and a corresponding edge of the second end face 1b. 
The conventional dielectric filter having the above-described structure is mounted on the surface of a mount board by soldering in such a way that the mount surface A on which one end portion of the input/output electrode 4 is disposed is put upon the surface of the mount board.
In this type of dielectric filter, a TE mode spurious occurs which has a frequency fs determined by a specific dielectric constant εr of the dielectric block, the cross-sectional area of the dielectric block in a direction parallel to both the axial direction (up and down direction) of the through hole and the layout direction (lengthwise direction of the dielectric block) of the through holes. Given that H and W are respectively the height and length of the dielectric block in FIG. 7, then the cross-sectional area of the dielectric block is H×W. When this spurious frequency is close to a harmonic wave whose frequency is double or triple the resonance frequency of the dielectric resonator, the spurious frequency imparts an undesirable influence on the stable operation or so of the reception circuit system and the transmission circuit system. To avoid such an influence, the conventional dielectric filter should have the size of the dielectric block or the outside size of the dielectric filter changed to set the resonance peak of the spurious frequency apart from the harmonic wave.
One possible solution to this problem is to add a trap resonator (see, for example, JP(A)-2002-164708, page 10 and FIG. 15). FIG. 8 is an exploded perspective view of a dielectric filter 10 to which a trap resonator is added. In this case, the dielectric filter 10, a trapping dielectric resonator 36, a chip capacitor 35 and a connection terminal 37 are laid out on a dielectric mount board 30.
The dielectric filter 10 has four dielectric resonators and a pair of input/output electrodes 4 capacitively coupled to the dielectric resonators of the input and output stages on the open end face. The input/output electrodes 4 extend on the lateral face (bottom side in FIG. 8) of the dielectric block of the dielectric filter 10 and insulated from the outer conductor 5. A ground conductor 33 and a pair of mount board input/output electrodes 31 insulated from the ground conductor 33 are formed on the top surface of the dielectric mount board 30. The outer conductor 5 of the dielectric filter 10 is connected to the ground conductor 33 on the top surface of the dielectric mount board 30 and the input/output electrodes 4 of the dielectric filter 10 are connected to the mount board input/output electrodes 31 on the top surface of the dielectric mount board 30.
The mount board input/output electrode 31 also serves as an electrode to couple with the trapping dielectric resonator 36 and has an extension portion 32 for the connection. One electrode of the chip capacitor 35 is connected to the extension portion 32 and the other electrode of the chip capacitor 35 is connected to the inner conductor of the trapping dielectric resonator 36 via the connection terminal 37. The outer conductor of the trapping dielectric resonator 36 is connected to the ground conductor 33 on the dielectric mount board 30.
Apparently, the addition of the trap resonator requires a lot of parts, such as the dielectric resonator, the chip capacitor and the connection terminal and needs a work like soldering to install the parts at predetermined positions. While the latter conventional case can suppress the spurious, therefore, it has difficulties in making the dielectric filter smaller and reducing the costs for the parts and the manufacturing cost.